The present invention relates to the field of fiberoptics and more specifically to the tunable filters for optical fibers and can be used for construction of wavelength-selective optical devices, based in particular on optoacoustic effect, with tunable spectral range of reflected (transmitted) electromagnetic waves.
The invention can be used for construction of an external resonator for semiconductor injection lasers and for multiplexing/demultiplexing in the WDM communication systems.
Different types of optoacoustic fiber-based devices are known, where wavelength-selective light conversion (such as the intensity reduction, phase modulation, frequency shift and mutual conversion of orthogonally polarized propagating waves) is accomplished by means of interference of said waves at the optical fiber inhomogeneities, which are induced by elastic deformation waves propagating along the optical fiber. Based on this optoacoustic effect, these deformations change the refractive index of the optical fiber, thus creating periodic structure traveling along the optical fiber. Interference of the waves of electromagnetic field guided within the optical fiber at this periodic inhomogeneities strongly depends on the correlation between the wavelength of the electromagnetic field wave and the wavelength of the elastic wave [see for example Yu. V. Gulyaev, M. Ya. Mesh, V. V. Proklov. Modulation effects in optical fiber waveguides and their applications. Moscow, Radio i Svyaz, 1991, in Russian]. In the known devices the elastic waves traveling in the optical fiber are induced (usually by means of piezoelectric transducer) in a straight-line piece of optical fiber between emitter and absorber of the elastic waves; both the emitter and the absorber are usually rigidly attached to the optical fiber.
The main disadvantage of all known optoacoustic optical fiber devices is the small length of optoacoustic interaction (less than 10-20 cm), the length being defined by attenuation of elastic waves. This length of interaction determines the coupling of unidirectional optoacoustically connected guided electromagnetic waves. The relatively short length of optoacoustic interaction results in wide spectral band of interaction of guided electromagnetic waves. Narrowing of the interaction spectral bandwidth by means of shortening of the elastic deformation wavelength (from 100-200 xcexcm to 0.5 xcexcm) does not seem to be very promising both technically (due to difficulties of exciting of 1 GHz frequency oscillations in optical fiber and stabilization of the frequency with 1 kHz precision) and physically (since elastic waves attenuation strongly increases as the frequency grows up).
A prior art device most similar to the device of the present invention is the all-optical fiber optoacoustical notch filter by Kim et al [U.S. Pat. No. 6,021,237 and H. S. Kim, S. H. Yun, I. K. Kuang et.al. All-fiber optoacoustic tunable notch filter with electronically controllable spectral profile. Opt. Lett., V.22, No.19,pp.1476-1478, 1997]. This filter consists of a straight-line piece of a single-mode optic optical fiber 15.5 cm long with flexural oscillations propagating along its axis. These oscillations are excited on one end of the piece by the cone-shaped emitter, surrounding the optical fiber. The flexural oscillations are absorbed on the other end of the piece in a special absorber. Frequency-selective transition of the IR light wave propagating along the optical fiber takes place due to electromagnetic wave interference on the periodic inhomogeneities of the refractive index. These inhomogeneities are caused by the propagating elastic wave via the optoacoustic effect. A symmetric single-cladding optical fiber with the 8.5 xcexcm diameter of the core is used in this device; the frequency of the elastic oscillation was varied from 1.8 MHz to 2.8 MHz; the center of the arising filtration band varied in the range from 1450 nm to 1650 nm. The wavelength of the elastic wave was about 650 xcexcm at the frequency of 2.33 MHz. The maximal damping of the propagating electromagnetic wave was xe2x88x9234 dB with 2 nm bandwidth.
The disadvantage of this device is its limited functional abilities caused by the short length of the interval of uniform elastic oscillations between the emitter and the absorber of the elastic waves (only about 15.5 cm). This limits the functional abilities to operation to the transmission mode only. Another shortcoming is the wide spectral range of filtration of about 1-2 nm.
The need exists therefore to increase the length of the optical fiber, which is subject to elastic oscillations between the emitter and the absorber thus enlarging the range of functional capabilities of the filter.
The principal object of the present invention is to provide a tunable optoacoustic filter with wide optoacoustic frequency-selective abilities by means of increasing the length of optoacoustic interaction in the interval of uniformity of the elastic oscillations. This elongation results in narrowing of the frequency bandwidth without the need to provide higher acoustic input power and in realization of new control regimes of the electromagnetic waves propagating along the optical fiber. In particular, one of the said regimes includes backward reflection of some part of the guided electromagnetic wave.
The object of the present invention is essentially achieved by coiling the optical fiber of the filter in a variety of ways. The principle of coiling allows to increase the usable length of the fiber to reach wider range of use. In its basic configuration, an optoacoustic frequency-selective tunable filter comprises at least one optical fiber, at least one emitter and at least one absorber of elastic deformation waves rigidly attached thereto via a cylindrical shell. The shell is made with at least one slit parallel to the shell""s axis, such that the optical fiber is placed next to and rigidly attached to the surface of the shell in a manner providing for easy transmission of elastic waves from the shell into the optical fiber and back. In this configuration, the fiber forms a spiral coil winding around the shell, whereas the emitter and the absorber of elastic waves are placed onto and attached to the opposite sides of the slit.
The cylindrical shell of the invention is preferably made thin and its radius is chosen to be large in comparison with the wavelength of the elastic wave excited by the emitter. The emitter of the elastic waves is made preferably from a piezoelectric material.
The absorber of elastic deformation waves is placed alongside the corresponding side of the slit so that it surrounds the fiber in the region where the optical fiber crosses the corresponding side of the slit.
It is also preferred to choose the material of the cylindrical shell and attach the fiber thereto such as to minimize the reflection and attenuation of the elastic wave energy on the interface between the optical fiber and the shell in the working range of frequencies of elastic oscillations of the filter.
A further objective of the present invention is to provide for total internal reflection on the interface of fiber cladding and the shell material, which preserves the major part of elastic deformation waves within the fiber and prevents dissipation of elastic motion energy outside the fiber. The shell material is preferably chosen in such a way as to ensure that the velocity of elastic waves (longitudinal and/or transversal) within the shell is higher than the velocity of the elastic waves (longitudinal and/or transversal, respectively) within the optical fiber cladding.
The technical result of the invention is the excitation of highly uniform running elastic wave in the entire section of the optical fiber wound around and attached to the cylindrical shell. This is in turn the result of uniformity of the elastic field in the shell outside the small region of the slit. The presence of at least one slit with the elastic wave absorber neutralizes the shell""s resonance features and minimizes backward reflection of the elastic waves from the corresponding side of the slit.
This design provides the best approximation of the elastic wave field in each revolution of the spirally wound optical fiber to the field propagating in a straight-line piece of the optical fiber as known and used in other optoacoustic devices.
An important advantage of the filter of the present invention is that the number of the optical fiber spiral revolutions around the shell is not limited. Therefore, the filter of the invention can provide excitation of almost uniform elastic running wave in a section of optical fiber of any desired length.
In the disclosed optoacoustic filter device, the piezoelectric transducer excites elastic oscillations of the optical fiber, which is wound in a coil around the cylindrical shell. The slit or slits in the shell are at least partially filled with the material strongly absorbing elastic waves (such as for example penopolyurethane or poured resin), which reduces the resonance character of excitation of the tangentially propagating oscillation waves of the shell. The emitter creates oscillation waves of elastic deformation directed longitudinally, transversely or at an angle along the shell and the optical fiber attached to the shell. The elastic deformation waves propagate along optical fiber in the plane normal to the axis of the shell in a highly regular and uniform fashion in a piece of optical fiber of any desired length.
This uniformity provides operation of the filter in the reflection mode, whereas the backward reflected waves are at least partly guided inside the optical fiber. This uniformity also reduces the spectral width of the filtered radiation in transition mode. The spectral band of filtration is tunable by adjustment of frequency and intensity of elastic oscillations in the optical fiber, in the same way as proposed in the similar devices of the prior art. Reflection and energy dissipation of the elastic wave on the interface between the optical fiber and the shell could be minimized by the choice of materials of the cylindrical shell 2 and the optical fiber 1, for example, using fused quartz as a material of the shell.
The above and other objects, aspects, features and advantages of the invention will be more readily apparent from the description of the preferred embodiments thereof taken in conjunction with the accompanying drawings and appended claims.